Integrated circuit directional coupler



March 10, 17970 I. 1'. Ho ET AL ZL ('h I) ZL v v ZL INVENTORS IRVING T. H0 SATISH K; MULLICK United States Patent INTEGRATED CIRCUIT DIRECTIONAL COUPLER Irving T. Ho, Poughkeepsie, N.Y., and Satish K. Mullick,

Kanpur, Uttar Pradesh, India, assignors to International Business Machines Corporation, Armonk, N.Y.,

a corporation of New York Filed Apr. 8, 1968, Ser. No. 726,269 Int. Cl. H01p 5/14 US. Cl. 333- 7 Claims ABSTRACT OF THE DISCLOSURE An integrated circuit directional coupler having two conducting lines separated from a semiconductor layer by an insulating coating on the semiconductor layer. A conducting ground plane is situated on the other side of the semiconductor. Coupling from one line to the other allows a small, efiicient, directional coupler to be constructed based on the transmission line.

BACKGROUND OF THE INVENTION This invention is in the field of integrated circuit directional couplers.

A directional coupler is a system containing a primary line and a secondary line arranged such that an incident voltage waveform into one end of the primary line is coupled to the other end of the primary line and to both ends of the secondary line in an amount dependent upon the structure of the coupler, the frequency of the wave form and the direction of travel of the wave.

Stripline directional couplers have been designed previously to include transmission lines separated from one or more ground planes by one or more layers of dielectric material.

SUMMARY OF THE INVENTION This invention is particularly concerned with a stripline directional coupler for use with integrated circuits. In the present invention it has been found that by separating a pair of parallel transmission lines from a ground plane with an insulator-coated semiconductor, the directional coupler thus constructed yields more ways of energy distribution than does the conventional stripline directional coupler with dielectric separation. The disclosed directional coupler, when excited at the rear end of the primary line, can provide an output to the near end or the far end of the secondary line, which is not possible with conventional directional couplers utilizing homogeneous dielectric media.

It is an object of this invention to provide a directional coupler mainly for use on integrated circuit chips.

It is a further object of this invention to provide a directional coupler using insulated semiconductor material as a base.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 illustrates the basic structure of the present invention; and

FIGURE 2 illustrates an equivalent circuit representation of the structure of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS bodlment, this semiconductor material 1s silicon although otherjemiconductor materials may be used. A layer 4 of insulating material is affixed to one side of wafer 5. In the preferred embodiment, this insulating material is silicon dioxide which can be formed by well-known techniques directly on the surface of silicon wafer 5. However, other insulating materials may be used and are within the scope of this invention. A layer 3 of conducting material forming a ground plane is aifixed to the other side of wafer 5. Two transmission lines 1 and 2 of conducting material are shown alfixed to the insulating layer 4. The conducting material is preferably metallic and may be deposited aluminum.

The thickness 1, width b, and spacing d of the transmission lines may be varied to vary the transfer function of the illustrated system. The ordinary transmission line in this system is 1 mil in width b and 1 micron in thickness 1. The thickest preferred line is 2 microns thick and could be 2 mil wide.

The transmission line illustrated in FIGURE 1 is especially suited for use as a directional coupler in an integrated circuit. A single line on top of an integrated circuit chip has a virtual ground plane located in the semiconductor substrate but near the SiO -Si boundary in determining the transmission line capacitance. However, in determining the transmission line inductance, the metallic ground plane such as layer 3 in FIG. 1 serves as the current return path. The capacitance is generally large because the oxide layer is very thin and the inductance is also large because the thickness of the semiconductor substrate may be quite large in comparison with the transmission line width b. As a result, a single transmission line on IC chip will have a large propagation delay because it has a large capacitance and a large inductance. Also as a result, two adjacent parallel lines on IC chips will have a negligible capacitive coupling by choosing h d and a large inductive coupling by choosing h d. In other words, the coupling between two adjacent lines on IC chip is dominated by inductive couplings. The line as illustrated has a large inductance and capacitance which causes a large delay on the line. This extra delay allows an effective directional coupler to be constructed with a much shorter transmission line than is possible with conventional transmission lines.

FIGURE 2 illustnates an equivalent transmission line circuit useful in analyzing the ellect of using the structure of FIGURE 1 as a directional coupler. Although the structure of FIGURE 1 is generally useful as a directional coupler, it is possible to vary the effectiveness of the coupling by varying the parameters of the structure.

There are two coupling coefficients between two transmission lines, namely the capacitive coupling coeflicient k and the inductive coupling coefficient k These coefficients are defined as follows 0 invention, there is a slow velocity wave and a fast velocity wave. The delay in the directional coupler appears different to the two waves of different velocity.

-Because-of the symmetry of the two lines of the directional coupler, one may. assume that the self-inductance of the two lines are equal (i.e., L =L and that the self-capacitance terms of the two lines are equal (i.e., C =C Define terms C and L such that 11= 22 and where t is time, and assuming that k k the following three equations can be derived to show the transfer functions of the directional coupler in each of the three possible directions:

The following equations define additional terms used in the transfer function equations:

w=21r times input frequency whea 1T1 As: A1 A3 hf: A1 \3 I which is the characteristic impendanoe of the line Y is the admittance of the slow wave mode Y; is the admittance of the fast wavemode.

and where r v1+k L EXAMPLE Assume a directional coupler with Z=. 0.05 meter L=1O nh./cm.= 1 0- henr'y/m. 7

0 4 pf./cm. =4 10- farad/m.

Then

If the chosen frequency is 560 mHz., then 6 (t /ITO: 22.4w sec./m.

5 By varying the amount of coupling as indicated by k the voltage ratios as just computed can be variable. The following table shows examples of output for a few typical cases with unit amplitude input.

6 '5. A directional coupler according to claim 4 wherein said conducting lines and said conducting plane are metallic.

6. A directional coupler according to claim 4 wherein 112 112 (l) I m (Z) In. alarm/(L0 (1+k1.) Bzl wh/(LC) (Z-kL) 21 (0) 01(0) m 0) 411' 21: 0 0 1 21r 1r 0 1 0 35 2 1 2 0. 5 0.87 o 3. 2r 0. 21w About 0.7 About 0.7 About 0 What is claimed is: said conducting lines and said conducting plane comprise 1. A directional coupler comprising: thin layers of aluminum. (a) a semiconductive layer, 7. A directional coupler usin the transmission line of (b) a thin insulating coating on said layer,

(c) two substantially parallel conducting lines situated adjacent to said coating on the side away from said layer, said lines being spaced apart a distance which is substantially greater than the thickness of said insulating coating but substantially less than the thickness of said semiconductor layer, and

(d) a conducting plane situated adjacent to said layer on the side away from said coating.

2. A directional coupler according to claim 1 wherein said conducting lines and said conducting plane are metallic.

3. A directional coupler according to claim 1 wherein the semiconductor of said semiconductor layer comprises silicon.

4. A directional coupler according to claim 1 wherein said thin insulated coating comprises a layer of silicon dioxide formed on the surface of said silicon.

claim 1 wherein a first signal is applied to one end of one of said conducting lines and a substantially proportional second voltage signal appears on at least one end of the other of said conducting lines.

References Cited UNITED STATES PATENTS 2,951,218 8/1960 Arditi 333-84 XR 3,370,184 2/1968 Zuleeg 307298 XR 3,416,042 12/1968 Thomas et a1 317-234 H. K. SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner U.S. Cl. X.R. 317-101, 234

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. Z55 Dated March 10, 1970 Inventor(g) Irving T. HO et 8.1

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 47 "rear end" should be --ne r end-- Column 3, line 23 should becu Column 3, line so a -{ff should be B s-ML Z Column 3, line 69 "wavemode" should be --wave mode-- Column 4, line 6 +T should be 41 3 Column 4, line 34 4fshould be {ff-"- Column 4, line 46 A 4 1 should be A Column 4, line 47 (31+)) should be (R R Column 4, line 56 =1. should be =1. '1

Column 5-6 In table heading 611% should be P 1= B l-= should be p I= m mzs m 1 mm: x. 8mm. ms Atteati om Commissioner or mums FORM PO-105O (10-69] UsCOMM-DC 50876-P69 e u s. covtauusm rnmrmc OFFICE: nu o-nl-an 

